1. Field of the Invention
The invention applies to radar systems generally and more specifically to inertialess electronic scan array arrangements permitting random beam positioning over as much as 360.degree. of azimuth coverage.
2. Description of the Prior Art
In the prior art, basic arrangements for beam scanning in a radar system are relatively well developed and known and their utility well established. The text "Radar Handbook" by Merrill I. Skolnik, (McGraw Hill 1970), provides a relatively current appraisal of the state of this art, generously supported by bibliographic references.
One of the oldest and most familiar methods of scanning a space in azimuth involves the use of a mechanically rotating directive antenna arrangement. Such expedients are widely used in so-called PPI radar systems, since they provide for the generation of beam patterns having required directivity and other characteristics, such as side-lobe level control. Moreover, that approach produces a beam pattern in space which is substantially of unvarying shape at all angles of scan. The principal disadvantage of the mechanically rotating antenna is, however, that it provides a very low data rate. Also, it does not have the capability of addressing any azimuth angle on a random basis, a feature which may be required for certain more advanced applications of radar systems.
Electronically scan arrays as a class generally can be made to fulfill all the requirements for rapid uniform beam shape scanning and for random angle address.
Obviously, a set of three or more planar phased arrays of known type can provide up to 360.degree. of scan, but such arrays inherently provide some beam distortion over their useful angles of scan.
The electronically scanned cylindrical array, however, is the logical choice for generating a beam pattern which does not distort with azimuth scan, and yet affords the speed and random address features and all the other flexibilities of inertialess scan. In the cylindrical array, the excited portion of the aperture may be rotated in synchronism with the beam to maintain the symmetry which preserves the beam pattern throughout the desired range of azimuth angles.
In the prior art, several network techniques are available for providing this synchronous rotation. In the so-called "Modal" approach, a complete Butler Matrix is used, together with a separate phase shifter for each circumferential element. That prior art approach is described in the technical literature, including an article by W. Korvin entitled "Latest Word in Space Talk; It Can Come From Anywhere" (Electronics Magazine PP 117-126, May 3, 1966), and also in an article by G. Shelet, entitled "Matrix Fed Cylindrical Array For Continuous Scanning", IEEE 1968, G-AP International Symposium Program and Digest, PP 7-9, September 1968.
Another approach, sometimes referred to as the "Switching-and-Phasing Technique" provides a large reduction in the number of phase shifters required because of the introduction of switching for coarse selection of the angular sector of interest, but still requires a relatively complicated sector-ordering matrix. That prior art approach has also been described in the technical literature, including an article by R. J. Gianini, entitled: "An Electronically Scan Cylindrical Array Based On A Switching-and-Phasing Technique", (IEEE 1969 G. A. P. International Symposium Program and Digest, PP 199-201, December 1969).
Additional discussion of prior art aspects of the equipment is included hereinafter in connection with the description of the preferred embodiments, so that reference to figures of the drawing may be made in that connection.
The manner in which the present invention provides for simpler and less costly instrumentation of the electronically scanned cylindrical array will be understood as this description proceeds.